The present invention relates to a system for transmitting first signals to, and receiving second signals from one or more remote transponders. More particularly, the invention relates to a radar system utilizing transponders which are capable of receiving an interrogating first signal, processing this signal and transmitting, in reply, a second signal that is derived from the first signal and contains encoded information.
Because the aforementioned encoded information normally includes an identification code which is unique to each transponder, and because the transponders of this type are relatively light weight and small and may be easily attached to other objects to be identified, the transponders are sometimes referred to as "labels". The entire system, including the interrogator/receiver apparatus and one or more passive transponders, is therefore often referred to as a "passive interrogator label system" or "PILS".
Passive interrogator label system of the type to which the present invention relates are disclosed in the following U.S. patents:
______________________________________ U.S. Pat. No. Patentee ______________________________________ 3,273,146 Horwitz, Jr. 3,706,094 Cole et al. 3,755,803 Cole et al. 3,981,011 Bell 4,058,217 Vaughan et al. 4,059,831 Epstein 4,263,595 Vogel ______________________________________
Such a system is also disclosed in the commonly-owned patent applications referred to above.
In general, a passive interrogator label system includes an "interrogator" for transmitting a first radio frequency signal; at least one passive transponder which receives this first signal, processes it and sends back a second radio frequency signal containing encoded information; and a receiver, normally located next to the interrogator, for receiving the second signal and decoding the transponder-encoded information.
In the aforementioned patent application Ser. No. 509,523, a passive interrogator label system is disclosed in which the interrogator transmits a first signal having a first frequency that successively assumes a plurality of frequency values within a prescribed frequency range. This first frequency may, for example, be in the range of 905-925 MHz, a frequency band that is freely available in many parts of the world for short range transmissions.
A passive (i.e., nonpowered) transponder associated with this system receives the first (interrogating) signal as an input and products a second (reply) signal as an output. Passive signal transforming means within the transponder, which converts the first signal to the second signal, includes:
(1) A multiplicity of "signal conditioning elements" coupled to receive the first signal from a transponder antenna. Each signal conditioning element provides an intermediate signal having a known delay and a known amplitude modification to the first signal.
(2) A single "signal combining element" coupled to all of the signal conditioning elements for combining the intermediate signals (e.g., by addition or multiplication) to produce the second signal. This second signal is coupled out to the same or a separate antenna for transmission as a reply.
The signal conditioning elements and the signal combining element impart a known informational code to the second signal which identifies, and is associated with, the particular transponder.
The receiving and decoding apparatus associated with the system includes apparatus for receiving the second signal from the transponder and a mixer, arranged to receive both the first signal and the second signal, for performing four quadrant multiplication of these two signals. The mixer produces, as an output, a third signal containing the difference frequencies (or frequencies derived from the difference frequencies) of the first and second signals, respectively.
Finally, the system disclosed in the aforementioned U.S. patent application Ser. No. 509,523 includes a signal processor, responsive to the third signal produced by the mixer, for detecting the phases and amplitudes of the respective difference frequencies contained in the third signal, thereby to determine the informational code associated with the interrogated transponder.
This particular system has a number of advantages over passive interrogator label systems of the type disclosed in the issued U.S. patents referred to above. For example, this system exhibits substantially improved signal-to-noise performance over the prior known systems. Also, the output of the signal mixer--namely, the third signal which contains the difference frequencies of the first (interrogating) signal and the second (reply) signal--may be transmitted over inexpensive, shielded, twisted-pair wires because these frequencies are in the audio range. Furthermore, since the audio signal is not greatly attenuated when transmitted over long distances, the signal processor may be located at a position quite remote from the signal mixer.
In practice, the transponders used in the passive interrogator label systems of the various types described above comprise surface acoustic wave ("SAW") devices which are susceptible to operational differences from transponder to transponder, depending upon manufacturing variations, and from moment to moment for a given transponder, depending upon variations in ambient temperature. In prior systems, such as those disclosed in the aforementioned patents to Cole et al., Vaughan et al. and Epstein, such variations are insignificant compared to the large "tap" delays inherent in the respective transponders. However, in the system disclosed in the aforementioned patent application Ser. No. 509,523, exceedingly small differences in tap delays are decoded into transponder identification numbers. Indeed, these delays are so small as to be in the order of magnitude of the changes in delay due to the manufacturing and temperature variations.
Variations in transponder response due to manufacturing tolerances are tracable to variations in the metallized pattern deposited on the piezoelectric substrate of the SAW device. Such a pattern includes SAW transducers, SAW reflectors and so-called SAW "delay pads". Variations in the position or interdigital finger line width of transducers and reflectors lead to variations in the time of propagation of a surface acoustic wave from its moment of launch to the moment its acoustic energy is reconverted into electrical energy.
More particularly, there are essentially two sources of manufacuturing variations: (1) variations tracable to the mask which is used in the photolithographic process to deposit the metallization, and (2) variations tracable to the photolithographic manufacturing process itself. Mask to mask variations can be minimized by using new masks only infrequently and by adjusting each new mask to include whatever imperfections existed in the prior mask. Manufacturing process changes can also be reduced, but cannot be eliminated entirely. For example, exposure times through a mask are extremely critical in determining the line widths of interdigital fingers. Furthermore, any lack of orthogonality of the mask during exposure results in differences in pattern dimensions from one side of the substrate to the other. As a result, manufacturing variations will occur during the manufacture of the SAW devices from lot to lot (e.g., about 20 wafers); from wafer to wafer (e.g., about 144 dies); from die to die; and even from one side of a die to the other.
Variations in the transponder response due to changes in temperature result, in part, from the thermal expansion of the substrate material. Although propagation distances are small, an increase in temperature of only 20 degrees Centigrade can produce an increase in propagation time by the period of one entire cycle at a transponder frequency of about 915 MHz.
Although both manufacturing and temperature variations may be ignored if the differences in surface acoustic wave propagation times between the various taps on a SAW device are large, the requirement of such large tap delay time differences leads to inefficient use of substrate real estate. As the size of a SAW device and, thus, the respective tap delay times are reduced, it becomes more and more desirable to find a mechanism for compensating both manufacturing and temperature variations. The need for compensation becomes even more critical when the transponder identification number is encoded by means of small changes in phase of the propagated surface acoustic wave. Such an encoding scheme is proposed in the aforementioned U.S. patent application Ser. No. 509,523.